How this instrument works
A rolling offset is what a pipefitter calls it when a pipe run needs to shift both sideways and vertically at once to get around an obstruction or connect two misaligned points — not a simple flat offset in one plane, but a diagonal jog using two matched fittings (commonly 45° or 90° elbows) connected by a single length of pipe run at an angle. The offset is the straight-line distance between the pipe's original centerline and where it needs to end up, measured perpendicular to the original run.
Two figures matter once the fittings and the offset distance are set: travel and run. Travel is the actual length of pipe to cut between the two fittings — the diagonal piece that does the actual shifting. Run is the horizontal setback, the distance measured along the original pipe's direction between where the first fitting sits and where the second one sits. Both come from basic right-triangle trigonometry: the offset is the side opposite the fitting angle, so travel (the diagonal, or hypotenuse) equals offset divided by the sine of the angle, and run (the adjacent leg) equals offset divided by the tangent of the angle.
Pipefitters lean on a handful of standard fitting angles — 45°, 30°, 60°, and occasionally others — because each has a well-known constant multiplier that turns the offset directly into travel and run without working the trigonometry from scratch on site. A 45° rolling offset, for instance, always has a travel equal to the offset times roughly 1.4142 (√2) and a run equal to the offset itself, since tan(45°) is exactly 1 — numbers experienced fitters often carry from memory rather than looking up.
- Enter the perpendicular shift the pipe run needs into Offset (in) — the straight-line distance between where the pipe starts and where it needs to end up.
- Enter the fitting angle into Fitting angle (deg) — 45° is the most common rolling-offset angle, using two 45° elbows.
- Read Travel (pipe length) for the actual length of pipe to cut and run between the two fittings.
- Read Run (setback) for the horizontal distance between where the first fitting sits and where the second one sits, measured along the original pipe direction.
Worked example — a 10-inch offset with 45° fittings
Enter 10 into Offset (in) and leave Fitting angle (deg) at its default of 45°, using a pair of standard 45° elbows. Travel (pipe length) reads 14.1421 in and Run (setback) reads 10.0000 in.
By hand: at 45°, tan(45°) = 1 exactly, so run = 10/1 = 10 in — the setback happens to equal the offset itself at this particular angle. And sin(45°) = √2/2 exactly, so travel = 10/(√2/2) = 10√2 ≈ 14.1421 in, the diagonal length of pipe to cut between the two elbows.
Questions
What's the difference between travel and run in a rolling offset?
Travel is the actual length of pipe you cut and install between the two fittings — the diagonal piece doing the shifting, and always the longer of the two figures for any angle under 90°. Run is the horizontal setback: how far along the original pipe's direction the second fitting sits from the first. A fitter cutting pipe needs travel; a fitter marking out where the second fitting lands on the wall or ceiling needs run.
Why is 45° the most common rolling-offset angle?
Because 45° fittings are stocked everywhere, and the math at exactly 45° is unusually clean — tan(45°) is exactly 1, so run always equals the offset itself, and travel is always the offset times √2 (roughly 1.4142), a constant many fitters memorize. Steeper or shallower angles are used when space constraints demand it, but 45° remains the default for a straightforward rolling offset.
Does a steeper fitting angle mean a longer or shorter travel piece?
A steeper angle (closer to 90°) means a shorter travel piece for the same offset, since sin(angle) grows toward 1 as the angle approaches 90°, shrinking offset/sin(angle). A shallower angle (closer to 0°) needs a much longer travel piece for the same offset, since sin(angle) shrinks toward 0 and the division grows large — which is why very shallow offsets sometimes call for a longer run of pipe than the offset distance might suggest.
Can I use this for a plain offset that isn't 'rolling'?
Yes — the same travel and run formulas apply to a simple offset that shifts a pipe run sideways in a single plane, not just a true rolling (diagonal, two-plane) offset. 'Rolling offset' specifically refers to combining a vertical and horizontal shift at once, but the underlying right-triangle trigonometry connecting offset, travel and run to the fitting angle is identical either way.
What angle should I use for a tight space where 45° fittings won't fit?
Shallower angles like 22.5° or 11.25° use less lateral offset relative to the pipe run's length but need considerably more travel and run for the same offset distance, since both offset/sin and offset/tan grow as the angle shrinks. Check what fitting angles are actually available in your pipe material and size before planning around one — not every angle is stocked in every fitting type.